In recent years almost all kinds of fibers have been processed by various methods to produce felts.
Examples of the fiber are, for instance, staple fibers of polyester, meta-linked aramid, para-linked aramid, polyphenylene sulfide (PPS), polyimide and the like. The felts are produced by placing a web obtained from the mentioned staple fibers on one side of a base fabric such as a plain-woven fabric made from monofilaments, multifilaments or processed yarns of the respective fiber or sandwiching the base fabric with the webs and then, needle-punching. Those felts are used in many industrial fields, for example, for applications such as sliding member, water repellent member, non-sticking member, member for supplying mold releasing agent in electrophotographic apparatuses and filter cloth for dust collection.
Particularly when the felts are used for filter clothes for dust collection, as compared with woven fabrics having the same dust collecting efficiency, the felt has a surface porosity of about two times, and there is a merit such that air permeability per unit filtrating area becomes two times and pressure loss is small and power consumption for air blasting is small. On the other hand, in case of the filter cloth for dust collection which is produced from a felt obtained by simple needling of the above-mentioned staple fibers, there is a problem that a part of particles to be removed enters inside of the felt during use, which causes blockage of the felt and increase in pressure loss. Also there is a problem that shaking down of adhered and accumulated particles on the surface of the felt are insufficient.
In order to solve these problems, with respect to felts used for filter clothes for dust collection such as felts for bag filter, various methods for improvements mentioned below have been proposed.
(1) A method in which fluffy fibers existing on a dust-collecting surface of the felt are minimized by singeing the fluffs with a burner or calendering the surface of the felt at a temperature close to its softening temperature; PA1 (2) A method in which needle punching is carried out to increase a density of a dust-collecting surface of the felt; PA1 (3) A method in which a filter cloth for dust collection obtained by laminating a non-woven fabric sheet on a surface of a needled felt is used in order to reduce blockage while maintaining strength and form of the felt, the method being disclosed in JP-A-3-60712; PA1 (4) A method in which a dispersion containing PTFE particles is applied on a dust-collecting surface of the felt, followed by fusing: and PA1 (5) A method in which a porous PTFE film is laminated on a dust-collecting surface of the felt with an adhesive.
However in the above method (1), dust particles accumulated on the dust-collecting surface of the felt become easy to be shaken down to a certain extent but it is not enough. In the above methods (2) and (3), a density of the dust-collecting surface of the felt increases (densification of surface), entering of dust particles into the inside of the felt can be prevented and the dust particles become easy to be shaken down, but the methods are still not satisfactory.
In the above method (4), since expensive PTFE is used only on the surface of the felt, it is advantageous from the viewpoint of cost. Also though shaking down of the dust particles becomes easy because of non-sticking property which PTFE possesses, a coating film of PTFE is easy to be separated and is inferior in friction property. Further in order to form the PTFE coating film, it is necessary to sinter at high temperature and thus, heat resistance is required for fibers which form the felt. Accordingly there is a problem that kind of fibers to be used is limited.
In the above method (5), there are the same merits as in the above method (4). However, since the adhesive is used, there is a problem that a porosity of the felt decreases and a pressure loss increases.
The above methods (1) to (5) direct to improvements in entering of dust particles and shaking down of the particles by densification and multilayered construction of the felt, but are not satisfactory because those methods have the mentioned problems as it is clear from the results obtained from microscopic observation of sectional view of dust-collecting surface of the felt.
An object of the present invention is to provide various members such as a filter cloth for dust collection which has small pressure loss, assures easy shaking down of dust particles and is excellent in friction property and mechanical strength; a sliding member having excellent sliding property; a water repellent member having excellent water repellency; a non-sticking member having excellent non-sticking property; and a member for supplying mold releasing agent in electrophotographic apparatuses which is excellent in air permeability, heat resistance and oil resistance, and to provide a multilayered felt used therefor and a process for producing the felt.